Solid terephthalic acid composition

ABSTRACT

A solid terephthalic acid composition and a process for producing terephthalic acid from para-xylene. The process comprises forming a mixture comprising the para-xylene, a solvent, a bromine source, and a catalyst; and oxidizing the para-xylene by contacting the mixture with an oxidizing agent at oxidizing conditions to produce a solid oxidation product comprising terephthalic acid, para-toluic acid, 4-carboxybenzaldehyde. The solvent comprises a carboxylic acid having from 1 to 7 carbon atoms and an dialkyl imidazolium ionic liquid; and the catalyst comprises at least one of cobalt, titanium, manganese, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, and zirconium. The solid terephthalic acid composition comprises, less than about 4,000 ppm-wt 4-carboxybenzaldehyde content, and more than about 2,000 ppm-wt a para-toluic acid.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/360,247 which was filed on Jun. 30, 2010.

FIELD OF THE INVENTION

This invention relates to solid terephthalic acid compositions andprocesses for producing terephthalic acid from a feed stock comprisingpara-xylene. More particularly, the invention relates to the oxidationof para-xylene in the presence of a solvent comprising a carboxylic acidand an ionic liquid.

BACKGROUND OF THE INVENTION

The oxidation of alkyl aromatic compounds, e.g., toluene and xylenes areimportant commercial process. A variety of oxidation products may beobtained including aromatic carboxylic acids such as terephthalic acid(1,4-benzenedicarboxylic acid) which is used, for example, in thepolymer industry.

U.S. Pat. No. 2,833,816 discloses processes for oxidizing aromaticcompounds to the corresponding aromatic carboxylic acids. A process forthe liquid phase oxidation of alkyl aromatic compounds uses molecularoxygen, a metal or metal ions, and bromine or bromide ions in thepresence of an acid. The metals may include cobalt and/or manganese.Exemplary acids are lower aliphatic mono carboxylic acids containing 1to 8 carbon atoms, especially acetic acid.

U.S. Pat. No. 6,355,835 discloses a process for the preparation ofbenzene dicarboxylic acids by liquid phase oxidation of xylene isomersusing oxygen or air by oxidising in the presence of acetic acid assolvent, cobalt salt as catalyst and an initiator. The oxidation step isfollowed by flashing the said reaction mixture to remove volatilesubstances and cooling and filtering to get crude benzene di-carboxylicacid as a solid product and filtrate. Recrystallizing the crude benzenedi-carboxylic acid to obtain at least 99% purity and recycling of thefiltrate are also disclosed.

U.S. Pat. No. 7,094,925 discloses a process for the oxidation of analkyl-aromatic compound, wherein the aromatic compound is admixed withan oxidising agent or sulfur compound in the presence of an ionicliquid. Air, dioxygen, peroxide, superoxide, any other form of activeoxygen, nitrite, nitrate, nitric acid or other oxides (or oxyhalides) ofnitrogen (hydrate or anhydrous) are preferably used as the oxidisingagent. The process is usually under Bronsted acidic conditions. Theproduct of the oxidation reaction is preferably a carboxylic acid orketone or an intermediate compound in the oxidation such as an aldehyde,or alcohol. The oxidation is preferably performed in an ionic liquidcontaining an acid promoter such as methanesulfonic acid.

US 2009/0326265 A1 discloses a process for preparing an aromaticpolycarboxylic acid by liquid phase oxidation of a di- ortri-substituted benzene or naphtalene compound, the process comprising astep of contacting the aromatic compound with an oxidant in the presenceof a carboxylic acid solvent, a metal catalyst and a promoter in areaction zone, wherein the promoter is an ionic liquid comprising anorganic cation and a bromide or iodide anion. Advantages of this processinclude high conversion without severe corrosion problems otherwiseassociated with halogen-containing compounds as promoter. The processdoes not necessitate the use of special corrosion-resistant material orliners in the process equipment; thus offering savings on investment andmaintenance costs and increasing plant reliability. The process of theinvention is especially suited for production of terephthalic acid fromp-xylene.

It is also known in the art that the oxidation products such as aromaticaldehydes, aromatic alcohols, aromatic ketones, and aromatic carboxylicacids may solidify or crystallize at oxidation conditions and/or as thereaction mixture cools. Thus, mixtures of oxidation products may beproduced which require further processing to increase the purity of thedesired product. In the production of terephthalic acid, the oxidationproduct is often referred to as crude terephthalic acid as it containsimpurities including color bodies and intermediate oxidation productsespecially 4-carboxybenzaldehyde (4-CBA). To obtain polymer grade orpurified terephthalic acid, various purification steps are known in theart including: washing the crude terephthalic acid with water and/or asolvent, additional oxidation or crystallization steps, and reacting asolution of dissolved crude terephthalic acid with hydrogen athydrogenation conditions usually including a catalyst comprisingpalladium and carbon. Often several purification steps are used.

U.S. Pat. No. 7,692,036 discloses an optimized process and apparatus formore efficiently and economically carrying out the liquid-phaseoxidation of an oxidizable compound. Such liquid-phase oxidation iscarried out in a bubble column reactor that provides for a highlyefficient reaction at relatively low temperatures. When the oxidizedcompound is para-xylene and the product from the oxidation reaction iscrude terephthalic acid (CTA), such CTA product can be purified andseparated by more economical techniques than could be employed if theCTA were formed by a conventional high-temperature oxidation process.

There remains a need in the art for alternate processes that produceterephthalic acid. In addition, processes that produce terephthalic acidand terephthalic acid compositions that are less costly and timeconsuming to purify are desirable. Terephthalic acid compositions havingdifferent ratios of contaminants may provide new intermediates useful asraw materials in other applications.

SUMMARY OF THE INVENTION

The invention provides new terephthalic acid compositions. In anotheraspect, the invention is a process for the oxidation of para-xylene toterephthalic acid. It has been discovered that the invention may be usedto produce solid terephthalic acid compositions having different amountsof contaminants relative to those observed in conventional processes. Inan embodiment, the invention provides terephthalic acid compositionsthat are less costly to purify. In another embodiment, the inventionproduces polymer grade or purified terephthalic acid without the use ofcatalytic hydrogenation.

In an embodiment, the invention is a process for producing terephthalicacid from para-xylene, the process comprising forming a mixturecomprising the para-xylene, a solvent, a bromine source, and a catalyst;and oxidizing the para-xylene by contacting the mixture with anoxidizing agent at oxidizing conditions to produce a solid oxidationproduct comprising terephthalic acid, 4-carboxybenzaldehyde, andpara-toluic acid. The solvent comprises a carboxylic acid having from 1to 7 carbon atoms and an dialkyl imidazolium ionic liquid; and thecatalyst comprises at least one of cobalt, titanium, manganese,chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, andzirconium.

In another aspect, the invention is a solid terephthalic acidcomposition comprising terephthalic acid, para-toluic acid, and4-carboxybenzaldehyde; wherein the composition has a4-carboxybenzaldehyde content of less than about 4,000 ppm-wt, and apara-toluic acid content of more than about 2,000 ppm-wt. In anembodiment, the solid terephthalic acid composition has a4-carboxybenzaldehyde content of less than about 100 ppm-wt, and apara-toluic acid content of more than about 2,000 ppm-wt.

DETAILED DESCRIPTION OF THE INVENTION

In general, the invention relates to terephthalic acid compositions andprocesses for the oxidation of para-xylene to terephthalic acid. Inbroad terms, the invention is a process for producing terephthalic acidfrom para-xylene comprising forming a mixture comprising thepara-xylene, a solvent, a bromine source, and a catalyst; and oxidizingthe para-xylene by contacting the mixture with an oxidizing agent atoxidizing conditions to produce a solid oxidation product comprisingterephthalic acid, 4-carboxybenzaldehyde, and para-toluic acid.

Para-xylene may be supplied to the process as a pure feed stream or thefeed stream may also include other compounds. In an embodiment, the feedstream has a para-xylene content of at least 98 wt %. In anotherembodiment the feed stream has a para-xylene content of at least 99 wt%. As the oxidation reaction generally proceeds through successivedegrees of oxidization, suitable feed compounds also include partiallyoxidized para-xylene compounds. Examples include para-toluic acid,4-carboxybenzaldehyde (4-CBA), terephthalic aldehyde, para-toluicalcohol, para-tolualdehyde, and 4-carboxybenzylalcohol. In anembodiment, at least 98 wt % of the feed stream is para-xylene andpartially oxidized para-xylene compounds.

In addition to para-xylene, the mixture comprises a solvent, a brominesource, and a catalyst. The solvent comprises a carboxylic acid havingfrom 1 to 7 carbon atoms and an ionic liquid. In an embodiment, thecarboxylic acid comprises acetic acid. The solvent may contain more thanone carboxylic acid. For example the solvent may further comprisebenzoic acid. In another embodiment, the carboxylic acid of the solventis acetic acid.

The solvent also comprises an ionic liquid. Relative to conventionalprocesses the amount of the carboxylic acid solvent is decreased whenionic liquids are used to avoid excessive solvent volumes. Generally,ionic liquids are non-aqueous, organic salts composed of ions where thepositive ion is charge balanced with negative ion. These materials havelow melting points, often below 100° C., undetectable vapor pressure andgood chemical and thermal stability. The cationic charge of the salt islocalized over hetero atoms, such as nitrogen, phosphorous, sulfur,arsenic, boron, antimony, and aluminum, and the anions may be anyinorganic, organic, or organometallic species.

Ionic liquids suitable for use in the instant invention include dialkylimidazolium ionic liquids. More than one ionic liquid may be used andadditional ionic liquids may, but are not required to be dialkylimidazolium ionic liquids. Dialkyl imidazolium ionic liquids have acation comprising two alkyl groups extending from a five member ring ofthree carbon and two nitrogen atoms. In an embodiment, the alkyl groupscontain from one to eight carbon atoms. There is no need for the twoalkyl groups to be the same or have the same number of carbon atoms. Inan embodiment, the ionic liquid cation is selected from the groupconsisting of 1-butyl 3-methyl imidazolium, 1-hexyl 3-methylimidazolium, and combinations thereof.

In another embodiment, the ionic liquid comprises an anion selected fromthe group consisting of halides, acetate, and combinations thereof. Theionic liquid may be selected from the group consisting of 1-butyl3-methyl imidazolium acetate, 1-butyl 3-methyl imidazolium bromide,1-hexyl 3-methyl imidazolium acetate, 1-hexyl 3-methyl imidazoliumbromide, and combinations thereof. In an embodiment, the ionic liquid isone of 1-butyl 3-methyl imidazolium acetate, 1-butyl 3-methylimidazolium bromide, 1-hexyl 3-methyl imidazolium acetate, and 1-hexyl3-methyl imidazolium bromide. In another embodiment, the ionic liquidcomprises at least one of 1-butyl-3-methylimidazolium bromide and1-butyl-3-methylimidazolium acetate. In a further embodiment, the ionicliquid comprises at least one of 1-hexyl 3-methyl imidazolium acetateand 1-hexyl 3-methyl imidazolium bromide.

In an embodiment, the solvent has a ratio of ionic liquid to carboxylicacid ranging from about 1:10 to about 10:1 by weight. In anotherembodiment the ratio of ionic liquid to carboxylic acid ranges fromabout 3:10 to about 10:1 by weight. The ratio of ionic liquid tocarboxylic acid may range from about 5:10 to about 10:1 by weight.Optionally, the solvent may further comprise water. The water may beadded to the mixture or generated in the mixture during the oxidationprocess. In an embodiment, the amount of water ranges from about 0.01 wt% to about 5 wt %, relative to the weight of the carboxylic acid havingfrom 1 to 7 carbon atoms. The amount of water may range from about 0.1wt % to about 2 wt %, relative to the weight of the carboxylic acidhaving from 1 to 7 carbon atoms. In an embodiment, the ratio of solventto para-xylene in the mixture ranges from about 1.5:1 to about 6:1 byweight. The ratio of solvent to para-xylene may range from about 2:1 toabout 4:1 by weight.

The catalyst comprises at least one of cobalt, manganese, titanium,chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium andzirconium. In an embodiment, the catalyst comprises cobalt andmanganese. The metal may be in the form of an inorganic or organic salt.For example, the metal catalyst may be in the form of a carboxylic acidsalt, such as, a metal acetate and hydrates thereof. Exemplary catalystsinclude cobalt (II) acetate tetrahydrate and manganese (II) acetate,individually or in combination. In an embodiment, the amount ofmanganese (II) acetate is less than the amount of cobalt (II) acetatetetrahydrate by weight.

The amount of catalyst used in the invention may vary widely. Forexample, the amount of cobalt may range from about 0.001 wt % to about 2wt % relative to the weight of the solvent. In an embodiment, the amountof cobalt ranges from about 0.05 wt % to about 2 wt % relative to theweight of the solvent. The amount of manganese may range from about0.001 wt % to about 2 wt % relative to the weight of the solvent. In anembodiment, the amount of manganese ranges from about 0.05 wt % to about2 wt % relative to the weight of the solvent. In another embodiment, theratio of cobalt to manganese ranges from about 3:1 to about 1:2 byweight on an elemental metal basis.

Bromine sources are generally recognized in the art as being catalystpromoters and include bromine, ionic bromine, e.g. HBr, NaBr, KBr,NH₄Br; and/or organic bromides which are known to provide bromide ionsat the oxidation conditions, such as, benzylbromide, mono- anddi-bromoacetic acid, bromoacetyl bromide, tetrabromoethane, ethylenedi-bromide. In an embodiment, the bromine source comprises or consistsessentially of or consists of hydrogen bromide. The amount of hydrogenbromide may range from about 0.01 wt % to about 5 wt %, relative to theweight of the solvent. In another embodiment, the amount of hydrogenbromide ranges from about 0.05 wt % to about 2 wt %, relative to theweight of the solvent.

Optionally, the mixture may further comprise ammonium acetate. In anembodiment, the amount of ammonium acetate ranges from about 5 wt % toabout 25 wt %, relative to the weight of the solvent. The amount ofammonium acetate may range from about 10 wt % to about 20 wt %, relativeto the weight of the solvent.

In an embodiment, the mixture comprises para-xylene, a solventcomprising acetic acid, a dialkyl imidazolium ionic liquid, andoptionally water, a bromine source comprising hydrogen bromide, acatalyst comprising cobalt and manganese, and optionally ammoniumacetate. In another embodiment, the mixture comprises para-xylene, asolvent comprising acetic acid, a dialkyl imidazolium ionic liquidcomprising 1-butyl 3-methyl imidazolium acetate, and optionally water, abromine source comprising hydrogen bromide, a catalyst comprising cobaltand manganese, and optionally ammonium acetate. In further embodiment,the mixture comprises para-xylene, a solvent comprising acetic acid, adialkyl imidazolium ionic liquid comprising 1-butyl 3-methyl imidazoliumbromide, and optionally water, a bromine source comprising hydrogenbromide, a catalyst comprising cobalt and manganese, and optionallyammonium acetate. In an embodiment, the mixture comprises para-xylene, asolvent comprising acetic acid, a dialkyl imidazolium ionic liquidcomprising 1-hexyl 3-methyl imidazolium bromide, and optionally water, abromine source comprising hydrogen bromide, a catalyst comprising cobaltand manganese, and optionally ammonium acetate. In another embodiment,the mixture comprises para-xylene, a solvent comprising acetic acid, adialkyl imidazolium ionic liquid comprising 1-hexyl 3-methyl imidazoliumacetate, and optionally water, a bromine source comprising hydrogenbromide, a catalyst comprising cobalt and manganese, and optionallyammonium acetate.

Oxidation processes according to the invention may be practiced inlaboratory scale experiments through full scale commercial operations.The process may be operated in batch, continuous, or semi-continuousmode. The mixture described above may be formed in various ways. Theorder of addition of the mixture components (e.g. para-xylene, solvent,bromine source, and catalyst) is not critical. In an embodiment, two ormore components may be combined or mixed before being combined or mixedwith other components. At least a portion of the mixture provides aliquid phase, though dissolution of one or more of the mixturecomponents may not be complete at any or some time during the process.The liquid phase may be formed by mixing the components at ambientconditions. In another embodiment, the liquid phase is formed as thetemperature of the mixture increases to the oxidation temperature. Themixture may be formed prior to the oxidation step, in the same ordifferent vessel as that used in the oxidation step. In anotherembodiment, the mixture is formed in an oxidation reactor, e.g. addingvarious streams of the components individually and/or in combination toa continuous or semi-continuous oxidation reactor. The mixture, and/orvarious streams of the mixture components may be heated before they aremixed together.

Though many conventional alkyl aromatic oxidation processes aretypically conducted in a mixed phase, and often include three phases(e.g. solid, gas, and liquid), they are frequently referred to in theart as “liquid phase” oxidation processes because the oxidationconditions are maintained to provide at least a portion of the mixturein the liquid phase. It is also known in the art that the number ofphases present may vary over time during the process. Processesaccording to the instant invention may also be conducted in a liquidphase or mixed phase in a similar manner as known in the art.

Conventional, liquid phase oxidation reactors as known in the art may beused to practice the invention. Examples include vessels, which may haveone or more mechanical agitators, and various bubble column reactorssuch as those described in U.S. Pat. No. 7,692,036. It is also known todesign, operate, and control such reactors and the oxidation reactionfor the oxidation conditions employed including, e.g., the temperature,pressure, liquid and gas volumes, and corrosive nature of the liquid andgas phases where applicable. See, e.g. U.S. Pat. No. 7,692,036 and U.S.Pat. No. 6,137,001.

The process of the invention also comprises at least one oxidizing stepwherein the para-xylene is oxidized by contacting the mixture with anoxidizing agent at oxidizing conditions to produce a solid oxidationproduct comprising terephthalic acid, para-toluic acid, and4-carboxybenzaldehyde (4-CBA). The solid oxidation product may furthercomprise at least one of benzoic acid, terephthalic aldehyde,para-toluic alcohol, para-tolualdehyde, and 4-carboxybenzylalcohol. Inanother embodiment the contacting step also produces a mother liquorcomprising the solvent, the bromine source, and the catalyst.

Suitable oxidizing agents for the process provide a source of oxygenatoms to oxidize the para-xylene and partially oxidized para-xylenecompounds at the oxidation conditions employed. Examples of oxidizingagents include peroxides, superoxides, and nitrogen compounds containingoxygen such as nitric acids. In an embodiment, the oxidizing agent is agas comprising oxygen, e.g. air, carbon dioxide, and molecular oxygen.The gas may be a mixture of gasses. The amount of oxygen used in theprocess is preferably in excess of the stoichiometric amount requiredfor the desired oxidation reaction. In an embodiment, the amount ofoxygen contacted with the mixture ranges from about 1.2 times thestoichiometric amount to about 100 times the stoichiometric amount on amolar basis. Optionally, the amount of oxygen contacted with the liquidphase mixture may range from about 2 times the stoichiometric amount toabout 30 times the stoichiometric amount.

Oxidizing conditions generally include a temperature ranging from about125° C. to about 275° C. and a pressure ranging from about atmospheric,i.e. 0 MPa(g), to about 6 MPa(g) and a residence time ranging from about5 seconds to about 2 weeks. That is, the mixture has a temperature and apressure within these ranges and may be maintained within these rangesfor a period of time within the residence time range. In anotherembodiment, the temperature ranges from about 175° C. to about 225° C.;and the temperature may range from about 190° C. to about 235° C. In anembodiment, the pressure ranges from about 1.2 MPa(g) to about 6.0MPa(g); and the pressure may range from about 1.5 MPa(g) to about 6.0MPa(g). In a further embodiment, the residence time ranges from about 10minutes to about 12 hours. The oxidation temperature, pressure andresidence time may vary based on a variety of factors including forexample, the reactor configuration, size, and whether the process is,batch, continuous, or semi-continuous. An oxidation condition may alsovary based on other oxidation conditions. For example, use of aparticular temperature range may enable use of a different residencetime range.

In an embodiment, the oxidation product produced by the instantinvention may precipitate, crystallize, or solidify in a liquid phasemixture at the oxidation conditions and/or as the mixture cools. Othercompounds, including color bodies, and other oxidation products maysolidify with or be trapped in the solid oxidation product thus reducingthe purity of the desired product. In an embodiment, the mixturecomprises a liquid phase. The mixture may comprise a gas phase such aswhen the oxidizing agent is added as a gas. The mixture may comprise asolid phase e.g. a mixture component, an oxidation product, or aby-product fails to dissolve or solidifies in the mixture. In anembodiment, the mixture comprises a liquid phase, a solid phase andoptionally a gas phase. In another embodiment, the mixture comprises aliquid phase and a gas phase.

As noted above and discussed below, it has been discovered that theinvention may be used to produce a solid oxidation product havingdifferent amounts of contaminants relative to those observed inconventional processes. In addition, the invention provides new ways tocontrol the level of various contaminants in the solid oxidationproduct. In an embodiment, a process according to the invention furthercomprises forming the oxidation product as a solid, optionally at theoxidizing conditions, to produce the solid oxidation product and amother liquor. The solid oxidation product may be separated from themother liquor, i.e. the liquid phase, and the mother liquor of theprocess may be recycled and reused in the contacting step or other stepsof the process described below.

Processes according to the invention, may comprise additional oxidizingsteps. In an embodiment a second oxidation step includes a secondoxidizing temperature that is lower than the temperature of the firstoxidizing step. Processes according to the invention may includeadditional oxidizing steps of the invention as described herein, and/orthe invention may be combined with other oxidizing steps such asconventional oxidizing steps known in the art. Multiple oxidation stepsmay be conducted in series and/or parallel and may be combined withother process steps such as purification steps described herein.

In a sequential embodiment, the invention includes a second oxidationstep wherein a portion or all of the solid oxidation product, or themother liquor, or both the solid oxidation product and the mother liquorproduced in the first oxidation step forms a second mixture with asecond solvent, a second bromine source, and a second catalyst. Thesecond mixture is contacted with a second oxidizing agent at secondoxidizing conditions to produce a second solid oxidation productcomprising terephthalic acid, 4-carboxybenzaldehyde, and para-toluicacid. The second solvent comprises a carboxylic acid having from 1 to 7carbon atoms and a dialkyl imidazolium ionic liquid; and the secondcatalyst comprises at least one of cobalt, titanium, manganese,chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, andzirconium. The second solvent, second bromine source, second catalyst,and second oxidation conditions may individually or collectively be thesame or different from those of the first oxidation step. Optionally, aportion of the para-xylene may be included in the second mixture. Theoptional elements and optional steps described for the first oxidationstep above are equally applicable to this second oxidation step.

In a parallel embodiment, the invention further comprises a secondoxidation step wherein a second mixture comprising a portion of thepara-xylene, a second solvent, a second bromine source, and a secondcatalyst is formed. The second mixture is contacted with a secondoxidizing agent at second oxidizing conditions to produce a second solidoxidation product comprising terephthalic acid, 4-carboxybenzaldehyde,and para-toluic acid. The second solvent comprises a carboxylic acidhaving from 1 to 7 carbon atoms and the second catalyst comprises atleast one of cobalt, titanium, manganese, chromium, copper, nickel,vanadium, iron, molybdenum, tin, cerium, and zirconium. Optionally, thesecond solvent further comprises a dialkyl imidazolium ionic liquid. Thesecond solvent, second bromine source, second catalyst, and secondoxidation conditions may individually or collectively be the same ordifferent from those of the first oxidation step. The optional elementsand optional steps described for the first oxidation step above areequally applicable to this second oxidation step.

In another embodiment, the invention further comprises purifying thesolid oxidation product comprising terephthalic acid,4-carboxybenzaldehyde, and para-toluic acid, that is, a solidterephthalic composition. Purifying may comprise one or more additionalsteps to isolate and purify the solid oxidation product. Examples ofpurifying steps include: separating wherein a solid terephthaliccomposition is separated from the mother liquor or another liquid phasesuch as by filtration and/or centrifugation; washing wherein a solidterephthalic composition is washed, for example with water and/oranother solvent component; drying a solid terephthalic composition; andhydrogenation processes. Such additional processing steps have beendescribed in the general literature and are well known to those ofordinary skill in the art to be used in various combinations to purifysolid terephthalic acid compositions. See for example, the referencescited in this application and the art cited therein.

A purification step of instant invention may further comprise a one ormore solvent contacting steps. A solvent contacting step comprisescontacting a solid terephthalic composition such as a washed solidoxidation product with a second solvent comprising at least one ofwater, a carboxylic acid having from 1 to 7 carbon atoms, a dialkylimidazolium ionic liquid, and a mother liquor to produce a second solidterephthalic composition. Solvent contacting may leach impurities fromthe solid terephthalic composition, and/or the solid terephthaliccomposition may be partially or completely dissolved in the solvent.Solvent contacting conditions include a solvent contacting temperature.The solvent contacting temperature may be lower than the oxidationtemperature. In an embodiment, the solvent contacting temperature is atleast 20° C. lower than the oxidizing temperature. Solvent contactingmay be practiced for example in the one or more crystallizers thatfollow the oxidation reactor in some conventional processes. The secondterephthalic composition may solidify, precipitate, or crystallize inthe second solvent of the solvent contacting step. The secondterephthalic composition has a higher terephthalic acid content relativeto the terephthalic acid content of the solid terephthalic compositionintroduced to the solvent contacting step as at least some impuritieshave been reduced.

The terephthalic composition of the solid oxidation product made by theinstant invention may be purified by known methods including the use ofa hydrogenation step. In an exemplary embodiment, the solid oxidationproduct has a 4-carboxybenzaldehyde content low enough so that ahydrogenation step is not required. That is, the invention enablesproduction of polymer grade or purified terephthalic acid without theneed for a hydrogen step and the subsequent processing steps known inthe art. In an embodiment, a process according to the invention includesone or more purification steps that exclude hydrogenation steps. Thatis, the purifying process steps are selected from the group of processsteps consisting of washing, separating, drying, solvent contacting, andcombinations thereof.

In an embodiment, purifying the solid oxidation product comprisesseparating the solid oxidation product from the mother liquor andwashing the solid oxidation product. The washed solid oxidation productis contacted with a second solvent at solvent contacting conditionsincluding a solvent contacting temperature to produce a solidterephthalic acid composition. The second solvent is selected from thegroup consisting of the oxidation step mother liquor, a carboxylic acidhaving from 1 to 7 carbon atoms, a dialkyl imidazolium ionic liquid,water, and combinations thereof. In an embodiment, the solventcontacting step further produces a second mother liquor. Optionally, thesolid oxidation product is at least partially dissolved in the secondsolvent. In an embodiment, the solvent contacting step further comprisesforming the solid terephthalic acid composition at the solventcontacting conditions. Optionally, the invention further comprisesseparating the solid terephthalic acid composition from the secondmother liquor. The process may further comprise: washing the separated,solid terephthalic acid composition with water to produced a washedsolid terephthalic acid composition; and drying the washed solidterephthalic acid composition to produce a dried solid terephthalic acidcomposition.

In an embodiment, the second solvent is the oxidation step motherliquor. In another embodiment the second solvent is the oxidation stepmother liquor and the solvent contacting temperature is lower than theoxidation temperature.

Solid oxidation products produced by processes according to theinvention may have different composition than terephthalic acidcompositions produced by conventional processes. Without wishing to bebound by theory, it is postulated that the use of ionic liquidsaccording to the invention alters the solubility of terephthalic acidand/or at least some of the partially oxidized para-xylene intermediatesthus altering the relative amounts of partially oxidized para-xyleneintermediates which co-precipitate or co-solidify with the terephthalicacid. Thus, the invention enables simpler and less costly purificationstep(s) relative to conventional processes.

In another embodiment, the invention is a first solid terephthalic acidcomposition comprising terephthalic acid, para-toluic acid, and4-carboxybenzaldehyde; wherein the composition has a4-carboxybenzaldehyde content of less than about 4,000 ppm-wt and apara-toluic acid content of more than about 2,000 ppm-wt In anembodiment, the first solid terephthalic acid composition has aterephthalic acid content of at least about 80 wt %; the terephthalicacid content of the composition may be at least about 85 wt %,optionally at least about 95 wt %.

In an embodiment, the para-toluic acid content of the first solidterephthalic acid composition is more than about 4,000 ppm-wt. Inanother embodiment, the para-toluic acid content of the first solidterephthalic acid composition is more than about 10,000 ppm-wt. In afurther embodiment, the para-toluic acid content of the first solidterephthalic acid composition is more than about 70,000 ppm-wt.Optionally, the para-toluic acid content of the first solid terephthalicacid composition ranges from about 2,000 ppm-wt to about 14 wt %. Thepara-toluic acid content of the first solid terephthalic acidcomposition may be at least 15 times greater than the4-carboxybenzaldehyde content of the first solid terephthalic acidcomposition on a weight basis.

In another embodiment, the invention is a second solid terephthalic acidcomposition wherein the 4-carboxybenzaldehyde content of the first solidterephthalic acid composition is less than about 3,000 ppm-wt. In anembodiment, the para-toluic acid content of the second solidterephthalic acid composition is more than about 4,000 ppm-wt. Inanother embodiment, the para-toluic acid content of the second solidterephthalic acid composition is more than about 10,000 ppm-wt. In afurther embodiment, the para-toluic acid content of the second solidterephthalic acid composition is more than about 30,000 ppm-wt.Optionally, the para-toluic acid content of the second solidterephthalic acid composition ranges from about 2,000 ppm-wt to about 9wt %. The para-toluic acid content of the second solid terephthalic acidcomposition may be at least 15 times greater than the4-carboxybenzaldehyde content of the second solid terephthalic acidcomposition on a weight basis.

In another embodiment, the invention is a third solid terephthalic acidcomposition wherein the 4-carboxybenzaldehyde content of the first solidterephthalic acid composition is less than about 1,000 ppm-wt. In anembodiment, the para-toluic acid content of the third solid terephthalicacid composition is more than about 4,000 ppm-wt. In another embodiment,the para-toluic acid content of the third solid terephthalic acidcomposition is more than about 8,000 ppm-wt. In a further embodiment,the para-toluic acid content of the third solid terephthalic acidcomposition is more than about 15,000 ppm-wt. Optionally, thepara-toluic acid content of the third solid terephthalic acidcomposition ranges from about 2,000 ppm-wt to about 3 wt %. Thepara-toluic acid content of the third solid terephthalic acidcomposition may be at least 20 times greater than the4-carboxybenzaldehyde content of the third solid terephthalic acidcomposition on a weight basis.

In another embodiment, the invention is a fourth solid terephthalic acidcomposition wherein the 4-carboxybenzaldehyde content of the first solidterephthalic acid composition is less than about 400 ppm-wt.

In another embodiment, the invention is a fifth solid terephthalic acidcomposition wherein the 4-carboxybenzaldehyde content of the first solidterephthalic acid composition is less than about 100 ppm-wt. In anembodiment, the para-toluic acid content of the fifth solid terephthalicacid composition is more than about 4,000 ppm-wt. Optionally, thepara-toluic acid content of the fifth solid terephthalic acidcomposition ranges from about 2,000 ppm-wt to about 2 wt %. Thepara-toluic acid content of the fifth solid terephthalic acidcomposition may be at least 50 times greater than the4-carboxybenzaldehyde content of the fifth solid terephthalic acidcomposition on a weight basis.

EXAMPLES

The examples are presented to further illustrate some aspects andbenefits of the invention and are not to be considered as limiting thescope of the invention.

Example 1

Experimental procedure: In a fume hood, load a Parr reactor with thespecified amounts of components for the given experiment seal thereactor. The Parr reactor includes a gas distributor to disperse the gasthrough a 1.6 mm opening into the liquid, a mechanical gas entrainmentstirrer, and baffles to ensure thorough mixing. Install the Parr reactorin a heater assembly at room temperature and connect a gas supply lineto the reactor and a condenser to the reactor outlet. During operation,gases exit the reactor through the condenser then a trap, then aback-pressure regulator. Connect a safety vent having a rupture disk,and thermocouples to the reactor. Connect a cooling water recirculatorto the condenser and begin to recirculate cooling water. Pressure testthe Parr reactor at room temperature and 1.4 MPa(g) (200 psig) usingnitrogen until there is no decrease in pressure for 15 minutes. Set theback pressure regulator on the reactor outlet to the experimentalpressure and pressure test the reactor under nitrogen. Begin raising thereactor temperature to the experimental temperature under the nitrogenatmosphere. Always follow all instructions for the specific reactorincluding temperature and pressure limits. When the reactor reaches thedesired temperature begin adding air at the experimental rate andmonitor the reactor temperature and pressure for the duration of thetest. During the test, the air flow into the reactor is maintained at1250 or 2500 standard cm³ per minute, the pressure is maintained at 4.1MPa(g), and the stirrer is maintained at 1600 rpm. At the end of thetest shut off the heater, cut the air flow and allow the reactor tocool. When the reactor cools to less than about 35° C., open the backpressure valve, stop the cooling water, and remove and empty the reactorto obtain the solid oxidation product and mother liquor.

The mother liquor and products are filtered under vacuum to separate thesolids and liquid. The solids are then mixed with approximately 100 ccdeionized water at room temperature and decanted. The room temperaturedeionized water mixing and decanting is repeated two additional times. Afourth wash with deionized water is heated to approximately 95° C. for30 minutes and then filtered. The solids are dried at 80° C. for 8-24hours before analyzing.

Examples 2-9

Examples 2-9 were individual tests conducted using the equipment andprocedure given in Example 1. The components of the mixture, given ingrams, operating temperature, time, and air flow, and results are givenin Table 1.

Example 2 (Comparative): Conventional test run without ionic liquids todemonstrate the level of impurities made using conventional solventsunder standard oxidizing conditions.

Example 3: Same oxidizing conditions as Example 2 except ionic liquidswere substituted for some of the acetic acid. Incorporating ionicliquids significantly reduces the 4-CBA impurity, but causes higherlevels of p-toluic acid and benzoic acid.

Example 4: Repeat of Example 3 except the oxidizing temperature wasincreased from 200 to 215° C. Increasing the temperature significantlyreduced the 4-CBA and p-toluic acid content compared to Example 3, butcaused an increase in benzoic acid.

Example 5: Same oxidizing conditions as Example 3.1-butyl-3-methylimidazolium acetate was not used and the amounts ofacetic acid, ammonium acetate and 1-butyl-3-methylimidazolium bromidewere increased. Here, both the 4-CBA and p-toluic acid are significantlyreduced compared to the conventional test (Example 2). The benzoic acidlevel is still higher than Example 2, but lower than Example 3.

Example 6: Repeat of Example 3 except the oxidation time was reduced to6 hours, which resulted in higher 4-CBA and p-toluic acid impurities,and less benzoic acid.

Example 7: Repeat of Example 6 except ammonium acetate was not used.Using ammonium acetate significantly reduces the 4-CBA but results inhigher p-toluic acid.

Example 8: Modified mixture components, increase air flow to 2500standard cm³ per minute, increased oxidation temperature to 215° C. anddecreased oxidation time to 3 hours. Changes significantly reduce the4-CBA.

Example 9: Repeat of Example 8 except tetrabutylphosphonium bromide wasused instead of 1-butyl-3-methylimidazolium bromide and1-butyl-3-methylimidazolium acetate and ammonium acetate were not used.

TABLE 1 Example Number 2 3 4 5 6 7 8 9 Oxidation Temperature, ° C. 200200 215 200 200 200 215 215 Oxidation Time, hours 10 10 10 10 6 6 3 3Air Flow, standard cm³ per minute 1250 1250 1250 1250 1250 1250 25002500 Mixture Components, grams para-xylene 20.0 20.0 20.0 20.0 20.0 20.020.0 20.0 glacial acetic acid 100.0 44.0 44.0 60.0 44.0 44.0 50.0 80.0water 2 0.4 0.4 0.4 0.4 0.4 0.4 0.4 1-butyl-3-methylimidazolium acetate0 20.0 20.0 0 20.0 20.0 10.0 0 1-butyl-3-methylimidazolium bromide 016.0 16.0 20.0 16.0 16.0 20.0 0 tetrabutylphosphonium bromide 0 0 0 0 00 0 20.0 ammonium acetate 0 12.0 12.0 20.0 12.0 0.0 20.0 0 hydrogenbromide 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 cobalt (II) acetate tetrahydrate0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 manganese (II) acetate 0.6 0.6 0.6 0.60.6 0.6 0.6 0.6 Analysis of solid product terephthalic acid, wt % 98.597.8 98.9 99.9 96.4 97.0 99.9 98.4 para-toluic acid, ppm-wt 2,340 19,600620 412 33,364 13,964 727 1,753 4-carboxybenzaldehyde, ppm-wt 12,493 42487 316 1,480 15,476 76 13,418 benzoic acid, ppm-wt 167 1,742 5,001 421532 430 640 79 4-hydoxymethylbenzoic acid, ppm-wt 403 678 0 0 989 496 0253

1. A solid terephthalic acid composition comprising terephthalic acid,para-toluic acid, and 4-carboxybenzaldehyde; wherein the composition hasa 4-carboxybenzaldehyde content of less than about 4,000 ppm-wt, and apara-toluic acid content of more than about 2,000 ppm-wt.
 2. The solidterephthalic acid composition of claim 1 wherein the composition has aterephthalic acid content of at least about 80 wt %.
 3. The solidterephthalic acid composition of claim 1 wherein the para-toluic acidcontent is more than about 4,000 ppm-wt.
 4. The solid terephthalic acidcomposition of claim 1 wherein the para-toluic acid content ranges fromabout 2,000 ppm-wt to about 14 wt %.
 5. The solid terephthalic acidcomposition of claim 1 wherein the para-toluic acid content is at least15 times greater than the 4-carboxybenzaldehyde content on a weightbasis.
 6. The solid terephthalic acid composition of claim 1 wherein the4-carboxybenzaldehyde content is less than about 3,000 ppm-wt.
 7. Thesolid terephthalic acid composition of claim 6 wherein the para-toluicacid content is more than about 4,000 ppm-wt.
 8. The solid terephthalicacid composition of claim 6 wherein the para-toluic acid content is morethan about 10,000 ppm-wt.
 9. The solid terephthalic acid composition ofclaim 6 wherein the para-toluic acid content ranges from about 2,000ppm-wt to about 9 wt %.
 10. The solid terephthalic acid composition ofclaim 9 wherein the composition has a terephthalic acid content of atleast about 85 wt %.
 11. The solid terephthalic acid composition ofclaim 1 wherein the 4-carboxybenzaldehyde content is less than about1,000 ppm-wt.
 12. The solid terephthalic acid composition of claim 11wherein the para-toluic acid content is more than about 4,000 ppm-wt.13. The solid terephthalic acid composition of claim 11 wherein thepara-toluic acid content ranges from about 2,000 ppm-wt to about 3 wt %.14. The solid terephthalic acid composition of claim 13 wherein thecomposition has a terephthalic acid content of at least about 95 wt %.15. The solid terephthalic acid composition of claim 11 wherein thepara-toluic acid content is at least 20 times greater than the4-carboxybenzaldehyde content on a weight basis.
 16. The solidterephthalic acid composition of claim 1 wherein the4-carboxybenzaldehyde content is less than about 400 ppm-wt.
 17. Thesolid terephthalic acid composition of claim 1 wherein the4-carboxybenzaldehyde content is less than about 100 ppm-wt.
 18. Thesolid terephthalic acid composition of claim 17 wherein the para-toluicacid content is more than about 4,000 ppm-wt.
 19. The solid terephthalicacid composition of claim 17 wherein the para-toluic acid content rangesfrom about 2,000 ppm-wt to about 2 wt %.
 20. The solid terephthalic acidcomposition of claim 17 wherein the para-toluic acid content is at least50 times greater than the 4-carboxybenzaldehyde content on a weightbasis.